Convert RefSetCache to a proper class and to use a Map internally

Using a `Map` instead of an `Object` provides some advantages such as
cheaper ways to get the size of the cache, to find out if an entry is
contained in the cache and to iterate over the cache. Moreover, we can
clear and re-use the same `Map` object now instead of creating a new
one.

src/core/primitives.js

@@ -239,46 +239,41 @@ class RefSet {
   }
 }
 
-var RefSetCache = (function RefSetCacheClosure() {
-  // eslint-disable-next-line no-shadow
-  function RefSetCache() {
-    this.dict = Object.create(null);
+class RefSetCache {
+  constructor() {
+    this._map = new Map();
   }
 
-  RefSetCache.prototype = {
-    get size() {
-      return Object.keys(this.dict).length;
-    },
+  get size() {
+    return this._map.size;
+  }
 
-    get: function RefSetCache_get(ref) {
-      return this.dict[ref.toString()];
-    },
+  get(ref) {
+    return this._map.get(ref.toString());
+  }
 
-    has: function RefSetCache_has(ref) {
-      return ref.toString() in this.dict;
-    },
+  has(ref) {
+    return this._map.has(ref.toString());
+  }
 
-    put: function RefSetCache_put(ref, obj) {
-      this.dict[ref.toString()] = obj;
-    },
+  put(ref, obj) {
+    this._map.set(ref.toString(), obj);
+  }
 
-    putAlias: function RefSetCache_putAlias(ref, aliasRef) {
-      this.dict[ref.toString()] = this.get(aliasRef);
-    },
+  putAlias(ref, aliasRef) {
+    this._map.set(ref.toString(), this.get(aliasRef));
+  }
 
-    forEach: function RefSetCache_forEach(callback) {
-      for (const i in this.dict) {
-        callback(this.dict[i]);
-      }
-    },
+  forEach(callback) {
+    for (const value of this._map.values()) {
+      callback(value);
+    }
+  }
 
-    clear: function RefSetCache_clear() {
-      this.dict = Object.create(null);
-    },
-  };
-
-  return RefSetCache;
-})();
+  clear() {
+    this._map.clear();
+  }
+}
 
 function isEOF(v) {
   return v === EOF;